Which eyepieces work better with a fast newtonian telescope?
Simsim asked:
I heard that eyepieces that deliver great views in a long focal ratio refractor might perform poorly in a “fast” newtonian.
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I heard that eyepieces that deliver great views in a long focal ratio refractor might perform poorly in a “fast” newtonian.
What are the factors that need to be taken into consideration when choosing eyepieces for a “fast” newtonian?
- which eyepiece design works better?
- does the focal length of the eyepiece have any effect on image quality?
…
My telescope is a 203mm, f=1200mm Dob.
Thanks!
January 31st, 2009 at 12:06 pm
An f/6 Newtonian shouldn’t be a problem, since it’s not particularly fast. Focal ratios f/5 and less are the problem. Tele Vue is the only manufacturer which tests and manufactures its eyepieces at f/5, and even they recommend using a coma corrector for anything shorter.
February 1st, 2009 at 10:41 pm
Actually, that isn’t terribly fast. But you may find that edge quality gets sacrificed a little with eyepieces that are less than premium. That’s because the increased curvature of the elements in a relatively fast scope results in tolerance stackups that are more severe, which in turn leads to views that are not flat across the field for any but the best eyepieces, where tolerances are held more tightly. I don’t think there is an absolute correlation to focal length, although the shorter ones may just compound the problem more.
You may want to look into the Televue line of eyepieces. They provide a field-flattening tool called a paracorr. You lose a little field of view, and I think it alters the magnification slightly, but it’s probably worth it. These are good eyepieces.
February 3rd, 2009 at 5:18 am
My scope is a 254 mm, fl=1200 mm dob. So that’s an F 4.7 scope. The 1.25″ Sirius Plossls that came with it are fine. 10 mm and 25 mm. But all the low power wide field 2″ eyepieces i’ve tried require a coma corrector to retain focus across the field. I got to try out a TeleVue Paracorr, and it works great. I’d also like to try a Baader coma corrector, since they’re cheaper.
I have an Epic 5.1 mm eyepiece that does not work for me. But before you think it’s the scope, everyone i’ve shown it to likes it in both my scope and other’s scopes. And, one of the guys has something similar, only more expensive, and he likes it in any scope, but i do not. I simply can’t get focus. I conclude it’s something about my eye, not the scope.
The only problems i’m aware of are low power/wide field.